A Systematic Literature Review on Service Composition for People with Disabilities: Taxonomies, Solutions, and Open Research Challenges

Integrating smart heterogeneous objects, IoT devices, data sources, and software services to produce new business processes and functionalities continues to attract considerable attention from the research community due to its unraveled advantages, including reusability, adaptation, distribution, and pervasiveness. However, the exploitation of service-oriented computing technologies (e.g., SOC, SOA, and microservice architectures) by people with special needs is underexplored and often overlooked. Furthermore, the existing challenges in this area are yet to be identified clearly. This research study presents a rigorous literature survey of the recent advances in service-oriented composition approaches and solutions for disabled people, their domains of application, and the major challenges, covering studies published between January 2010 and October 2022. To this end, we applied the systematic literature review (SLR) methodology to retrieve and collate only the articles presenting and discussing service composition solutions tailored to produce digitally accessible services for consumption by people who suffer from an impairment or loss of some physical or mental functions. We searched six renowned bibliographic databases, particularly IEEE Xplore, Web of Science, Springer Link, ACM Library, ScienceDirect, and Google Scholar, to synthesize a final pool of 38 related articles. Our survey contributes a comprehensive taxonomy of service composition solutions, techniques, and practices that are utilized to create assistive technologies and services. The seven-facet taxonomy helps researchers and practitioners to quickly understand and analyze the fundamental conceptualizations and characteristics of accessible service composition for people with disabilities. Key findings showed that services are fused to assist disabled persons to carry out their daily activities, mainly in smart homes and ambient intelligent environments. Despite the emergence of immersive technologies (e.g., wearable computing), user-service interactions are enabled primarily through tactile and speech modalities. Service descriptions mainly incorporate functional features (e.g., performance, latency, and cost) of service quality, largely ignoring accessibility features. Moreover, the outstanding research problems revolve around (1) the unavailability of assistive services datasets, (2) the underspecification of accessibility aspects of disabilities, (3) the weak adoption of accessible and universal design practices, (4) the abstraction of service composition approaches, and (5) the rare experimental testing of composition approaches with disabled users. We conclude our survey with a set of guidelines to realize effective assistive service composition in IoT and cloud environments. Researchers and practitioners are advised to create assistive services that support the social relationships of disabled users and model their accessibility needs as part of the quality of service (QoS). Moreover, they should exploit AI/ML models to address the evolving requirements of disabled users in their unique environments. Furthermore, weaknesses of service composition solutions and research challenges are exposed as notable opportunities for future research.

Integrating smart heterogeneous objects, IoT devices, data sources, and software services to produce new business processes and functionalities continues to attract considerable attention from the research community due to its unraveled advantages, including reusability, adaptation, distribution, and pervasiveness. However, the exploitation of service-oriented computing technologies (e.g., SOC, SOA, and microservice architectures) by people with special needs is underexplored and often overlooked. Furthermore, the existing challenges in this area are yet to be identifed clearly. Tis research study presents a rigorous literature survey of the recent advances in service-oriented composition approaches and solutions for disabled people, their domains of application, and the major challenges, covering studies published between January 2010 and October 2022. To this end, we applied the systematic literature review (SLR) methodology to retrieve and collate only the articles presenting and discussing service composition solutions tailored to produce digitally accessible services for consumption by people who sufer from an impairment or loss of some physical or mental functions. We searched six renowned bibliographic databases, particularly IEEE Xplore, Web of Science, Springer Link, ACM Library, ScienceDirect, and Google Scholar, to synthesize a fnal pool of 38 related articles. Our survey contributes a comprehensive taxonomy of service composition solutions, techniques, and practices that are utilized to create assistive technologies and services. Te seven-facet taxonomy helps researchers and practitioners to quickly understand and analyze the fundamental conceptualizations and characteristics of accessible service composition for people with disabilities. Key fndings showed that services are fused to assist disabled persons to carry out their daily activities, mainly in smart homes and ambient intelligent environments. Despite the emergence of immersive technologies (e.g., wearable computing), user-service interactions are enabled primarily through tactile and speech modalities. Service descriptions mainly incorporate functional features (e.g., performance, latency, and cost) of service quality, largely ignoring accessibility features. Moreover, the outstanding research problems revolve around (1) the unavailability of assistive services datasets, (2) the underspecifcation of accessibility aspects of disabilities, (3) the weak adoption of accessible and universal design practices, (4) the abstraction of service composition approaches, and (5) the rare experimental testing of composition approaches with disabled users. We conclude our survey with a set of guidelines to realize efective assistive service composition in IoT and cloud environments. Researchers and practitioners are advised to create assistive services that support the social relationships of disabled users and model their accessibility needs as part of the quality of service (QoS). Moreover, they should exploit AI/ML models to address the evolving requirements of disabled users in their unique environments. Furthermore, weaknesses of service composition solutions and research challenges are exposed as notable opportunities for future research.

Introduction
Service composition enables the integration of services into a sophisticated digital service that provides new augmented business processes [1]. Tis topic continues to fascinate researchers and practitioners, particularly since the number of heterogeneous services and smart objects has risen exponentially. For instance, connected IoT devices are forecasted to reach 50 billion devices worldwide by 2030 [2]. Fortunately, paradigms of service-oriented architecture facilitate the combination of such interoperable components to produce added-value and adaptable services by solving scalability, performance, and security issues, among others [3]. Nonetheless, the empowerment of some particularly deprived user groups remains largely overlooked. Tis review searches the prospects and challenges of service composition for disabled persons.
Moreover, the number of people classifed to have some disability has exceeded one billion globally, equating to a staggering 15% of the world population [4]. Researchers emphasize the importance of making interactive systems and services accessible for users with disabilities. Accessibility is one of the pivotal guidelines of universal design [5]. Moreover, the notion of ambient assisted living (AAL) has emerged to reduce the technical barriers of using modern technologies by senior persons. However, the opportunities ofered by pervasive computing and artifcial intelligence remain far from being exploited to satisfy the technological needs of people with impairments.
Our survey is motivated by the lack of understanding of the practices, approaches, and solutions that support the seamless composition of services for persons with disabilities. Notably existing service composition surveys, such as [6][7][8], analyzed composition solutions and tools aimed at creating services for consumption by able-bodied users. Tus, people with disabilities are completely disregarded in these surveys. Important aspects, such as inclusive design practices and accessibility features, are not included in their analysis. Moreover, many of these surveys sufered from methodical shortcomings, such as nonadherence to SLR best practices. On the contrary, our work contributes an in-depth understanding of accessible service composition for people with disabilities, with a focus on the following objectives: (i) Develop a taxonomy of accessible service composition encompassing several aspects, such as assistive services, semantic annotations, composition approaches, and execution environments. Te taxonomy may be used to assist the designers of software services to understand and tackle the specifc requirements of people with disabilities (ii) Formulate insight into the accessible software services and target users addressed within the scope of service composition (iii) Provide a comprehensive synthesis of the pivotal service composition approaches for creating integrated services that people with disabilities can access (iv) Identify and summarize the gaps and open issues pertaining to the area of accessible service composition (v) Defne future research directions in service composition for disabled people We organized this systematic literature review into six sections. Section 2 sheds light on the service composition concepts and practices and sets out the motivation for the survey. Section 3 inspects the current surveys and SLRs to clarify the gaps in the literature. Section 4 details the search methodology and highlights the contributions of the survey. Section 5 presents the main fndings and discusses the possible implications. Section 6 elaborates on the research problems and acknowledges the limitations and threats to the validity of our survey.

Background Works
In this section, we remind of the fundamental concepts pertaining to the lifecycle of service composition. In the next section, we present examples of smart and assistive services developed to improve the quality of life of disabled users.

Te Key Ingredients.
Nowadays, with the rapid growth of technological advancements and the emergence of modern computing paradigms (e.g., pervasive computing [9], IoT [10], and Cloud [11] among others), heterogeneous services have become easily accessible, connectable, and integrable through dedicated web APIs (e.g., [12,13]), including web services, web applications or mashups, IoT services, mobile services, big data services, and machine learning applications.
Service-oriented architecture (aka SOA) consists of grouping services together, where a service provided by service providers represents the essential part of a service composition process. Te features of service composition can be changed by modifying services. A software engineering team typically selects the best collection of services to satisfy the functional and nonfunctional requirements of a system [14]. Service-oriented architecture can be viewed as a software development approach based on replaceable components with standardized interfaces for interaction over standardized protocols [15]. SOA encapsulates the implementation details from the rest of the components, enabling the combination and reuse of components to build complex software packages and ensuring independence from the platforms and development tools [16].

Te Foundations of Service Composition.
In a serviceoriented architecture, the general life cycle of service development consists of a myriad of phases, including service defnition, discovery, selection, invocation, composition, deployment, and monitoring [17][18][19]. Service composition is perhaps the most critical phase in the development cycle where newly added value is produced and ofered to potential consumers through the combination of atomic services [20].
Moreover, the service composition process incorporates the subsequent phases, as shown in Figure 1. Te frst phase is composition planning which aims to specify the requested service and decompose it into a set of tasks. Ten, we have the service discovery, which is a search for a service that matches the previously selected tasks. Next, a service is selected among several discovered candidates, and the tasks are executed [21].
Te composition process is inherently a challenging process and entails technical complexities. Terefore, signifcant research eforts have been exerted to understand and facilitate the combination of services by programmers [22] as well as nonprogrammers [23,24]. Moreover, various tools were developed to integrate data, services, and web resources [25]. To guide service selection for people with impairments, the authors in [26] proposed a machine learning-driven framework taking into account user context and disability factors. However, tools dedicated to empowering the development of accessible services are still rare.
Classically, the composition of services can be either manual or automatic. In manual service composition, an end-user programmer should create an abstract representation of the composition process [27]. In automatic service composition, a composite service specifcation can be generated automatically by giving a set of component services and a specifed requirement requested by the user [28]. Moreover, the assisted composition is proposed as a middleground solution between completely manual and automatic approaches [29], where the initial logic is created by a composer and some composition tasks (e.g., selection of compatible services for tasks) are guided by a dedicated intelligence-empowered tool. From another perspective, service composition could be classifed into static and dynamic composition [27]. In the static composition, the assembly of individual services is achieved at design time resulting in the creation of static services. On the other hand, in the dynamic composition, individual services are assembled at runtime to deliver a dynamically adaptive composite service based on user profle and context of use.

Ubiquitizing Accessible Services.
A related research strand that has gained considerable attention is interactive smart homes [30] and ambient intelligent environments to support the daily activities of the aging population [31]. More relevant to our survey, assistive technologies and smart services that accommodate the needs and constraints of disabled persons have emerged recently to facilitate their quick adoption of ICT [32,33]. An example of these assistive technologies includes context-aware health services and emergency systems [34]. Another application system employs speech recognition to empower disabled users to operate smart home devices and perform grocery shopping [35]. Typically, such systems incorporate a myriad of IoT devices and sensors (e.g., body position and mobility) that are connected to a federated cloud platform.
Ambient assisted living (AAL) systems refer to the concept of applying computational intelligence (e.g., machine learning) to a designated environment to enable the independent interaction and use of the available technologies and services by older persons [36,37]. For instance, a smart kitchen was designed to assist the elderly and people with cognitive and physical impairments to use their kitchen appliances (e.g., fridge, oven, washing machine, etc.) autonomously [38]. AAL is unique in its ability to provide assistive IoT technologies, irrespective of their complexity, such as wearable sensors, smart objects, robots, home appliances, mobile devices, and user interfaces for the elderly population [37]. However, it does not necessarily focus on the composition aspects of heterogeneous services and devices.
Assistive technologies and services may be designed to serve two distinct user groups, namely, the older and disabled users. However, these two groups difer with respect to several characteristics and needs. Senior people may experience a decline or loss of physical or cognitive abilities as they age, but it is not always the case. However, disabled persons are likely to have a higher and permanent degree of disability. Tey usually require constant assistance throughout their life. Since there are a handful of recent surveys on AAL, e.g., [39,40], the focus of our work is limited to the composition approaches for people with disabilities.
Despite their promising advantages, assistive technologies for disabled persons still encounter various challenges, including the creation of assistive technologies that support independent living [41]. Moreover, a recent survey calls upon the research community to (1) pay close attention to the adaptability of services to the new habits and behaviors of disabled people, (2) be aware of the current context of use and users, and (3) emphasize the need to apply the usercentered design methodology during the design of such services [37]. In the next section, we critically review the existing surveys and pinpoint the research gaps that we aim to tackle in this study.

Motivations for our SLR.
Our frst logical step was to justify the necessity for a new survey on service composition approaches for disabled users. Our initiative is motivated from two perspectives. First, several dispersed research efforts strived to develop SOA solutions that support the integration of services for disabled users. Tese works stem from the belief that such special user groups require unique user requirements and considerations than nondisabled users. Nondisabled people are defned as those who do not have any sort of disability or can perform their day-to-day tasks without needing any level of help, including technical assistance and/or human assistance. However, we could not fnd any survey that integrates these works into one place to help understand the SOA landscape. Terefore, we took the quest to synthesize service composition studies that target disabled people and shed light on the major research challenges in the area. Second, previous fndings (e.g., [5,42]) advocate developing accessible interactive services and systems to empower and facilitate universal design and Computational Intelligence and Neuroscience access. Accessible design refers to the inclusion of the needs of disabled persons (e.g., blind, deaf, etc.) into the software design process [43]. Te principles of accessible design also apply to SOA platforms and services. In other words, composite software services and mashups ought to cater to the abilities and constraints of people with various physical and cognitive limitations. Hence, our work is the frst efort to pave the way toward assistive service composition for disabled people. We seek to formulate an in-depth understanding of the approaches, algorithms, platforms, and languages of service composition tailored to ofer digital accessibility features to assist deprived users in consuming integrated interactive services (e.g., IoT services, web services, and smart services).

Summary of Previous Surveys.
We thoroughly investigated 13 recent surveys spanning the area of service composition with a particular focus on end-users who are regarded as "people with disabilities" irrespective of their type or degree of disability. Our goal was to identify research works summarizing existing service-oriented architecture approaches and solutions to create accessible digital services for users with various disabilities. We systematically searched acknowledged databases (e.g., ScienceDirect, Scopus, IEEE Xplore, and Google Scholar) to fnd relevant review studies and systematic literature surveys. We restricted our search to reviews and SLRs published on or after 2010 using diferent combinations of search keywords (e.g., "service composition," "survey," and "review") depending on the accepted syntax in each academic search engine. Next, we studied the retrieved surveys and systematic literature reviews to identify the missing research gaps and diferentiate our work from the past fndings. Te idea here was to develop a new understanding that extends the previous knowledge in the research area of SOA. Less relevant SOA surveys, such as [44][45][46], were excluded from the analysis. Table 1 compares the prominent surveys in the service composition territory on several metrics, including the focus of the survey, years covered, and major limitations. Tree main remarks can be observed in Table 1. First, none of the existing reviews and SLRs surveyed service composition solutions, approaches, and platforms for the disabled people. Second, most works shifted the focus and application of composition approaches from traditional web environments to cloud, Internet of Tings, and ambient intelligence. Tis is a natural move, given the widespread modern IT technologies. Tird, each survey reviewed approximately 20 to 42 articles published between 2003 and 2017. Some surveys missed essential details about their methodology, including the databases used and the number of articles.
As observed in Table 1, prominent SOA surveys were published between 2013 and 2022. Tese surveys reviewed service composition studies that were published from 2003 to 2017. Many of those surveys covered a publication period ranging from four to six years. In our case, we focused on the articles published between 2010 and 2022, a publication window that is greater than most other surveys in the service composition area. We intended to explore the recent computing approaches that were applied to create accessible SOA solutions. Moreover, our search methodology returned 38 relevant articles, which we believe are enough articles to answer our research questions and draw solid conclusions about accessible service composition. In fact, keeping our SLR focused on this reasonable number of articles enabled us to conduct an in-depth analysis of the selected studies.
Te topic of composing assistive services for disabled persons has received little attention, and research studies in this area are scarce. Terefore, to the best of our knowledge, this research is the frst survey to explore and synthesize service composition solutions for people with special needs, irrespective of their application domain. Other related areas, such as AAL, are not the main scope of this article since there are several recent AAL-specifc surveys already published, e.g., [39]. Moreover, our target population based on the PICO methodology (Table 2) is disabled people, irrespective of age.

Gaps in Literature
Findings. Based on the comparison of the prominent surveys in service composition (see Table 1), we infer three signifcant research gaps that helped us frame our research questions posited in the next section. In our view, previous works failed to address the following aspects:   Computational Intelligence and Neuroscience (iii) Research gap three: a substantial weakness in understanding the issues that hinder the integration of accessible services.
To summarize, our systematic survey endeavors to identify and synthesize evidence-based research works that explored composition approaches to simplify the creation of integrated services for consumption by various disability groups. Tese groups include users disadvantaged by visual and hearing impairments, cognitive impairments, and physical disabilities.

Systematic Literature Review Methodology
Typically, there are two approaches to reporting review fndings, namely, systematic mapping studies and systematic literature review [56]. Although the diferences between these two types are subtle, they can be linked to and justifed by the genre of the methodology applied by the researchers.
In systematic mapping studies (SMS), the researchers generally focus on structuring the research area broadly without evaluating the studies in detail, while systematic literature reviews (SLRs) identify, collect, and synthesize evidence about the main results to answer specifc research questions [57]. Hence, we could classify our research work between the two types because we provide a complete synthesis of the related works, yet we do not describe each study separately. We deliberately summarize the key fndings and highlight the distinguished studies in our topic of investigation.
Designing a precise review protocol is pivotal to the correctness and repeatability of a systematic literature review. Terefore, we closely followed and applied the reputable Kitchenham's guidelines in this SLR [58]. Figure 2 depicts three main phases of Kitchenham's methodology. As can be observed, the frst phase starts by arguing the need for this survey, followed by positing the specifc research questions and establishing the search strategy. Te second phase executes the search protocol and applies inclusion and exclusion criteria to determine the most relevant articles to the SLR. Te third phase documents the answers to the research questions and highlights the evidence from the selected articles.

Research Questions.
Te PICO search strategy was adopted to help construct the research questions for our SLR [59]. Te PICO model is well known for producing precise and extensive search results. PICO specifes four components, namely, population, intervention, comparison, and outcomes, as listed in Table 2. In summary, our SLR investigates service composition approaches (i.e., intervention) that support the creation of accessible and universal services (i.e., outcome) for users with disabilities (i.e., population). Te PICO elements helped us in formulating the search strings. We created equivalent search phrases for each academic database to guide the automated search during the search process.
Moreover, we used the PICO model to produce four motivating questions for our SLR as follows: ( Tis question summarizes the major problems that deter the development of accessible services for disabled users. Tese problems will serve as a practical road map for other researchers to focus their future works in a bid to satisfy the digital needs of people with disabilities.

Search Strategy and Process.
Te search process was initiated by scoping the search terms, which were derived from the PICO elements and aligned with the abovementioned research questions. As shown in Table 2, synonyms of our target population and intervention were used to expand our results, as recommended in [60]. Boolean operators (AND and OR) were used to link the search keywords and facilitate the search in the selected databases.
Te composite search terms were constructed and used to fnd potential articles in six academic databases, including ACM Digital library, IEEE Xplore, ISI Web of Science, ScienceDirect, Springer Link, and Google Scholar. Other bibliographic resources, such as DBLP and CiteSeer, were not searched since their results are already included within six major databases. Our protocol-driven searches looked up the title, abstract, and keywords of articles in all databases except for Google Scholar, where only the title was searched. Tis was because the full search of text in Google Scholar Computational Intelligence and Neuroscience would return a huge number of articles. Moreover, our search was restricted to service composition studies published between 2010 and 2022. Tis was when pervasive computing and IoT services gained momentum and popularity.
We kept our search focused on the last 12 years and applied a comprehensive and meaningful fltering mechanism considering the various SLR guidelines, resulting in a reasonable and manageable number of articles (38 selected articles). Tis enabled us to carry out a thorough analysis of each article. Moreover, several studies have mentioned ten years window as an appropriate period to retrieve the most relevant research articles in the feld of interest. For example, the study published by [61] uses a ten-year time frame, i.e., 1995-2005, as the most appropriate window to search for relevant articles. Similarly, a more recent study [62] also uses ten years as the most appropriate time frame to search for relevant studies.
Admittedly, the territory of service-oriented architecture for people with disabilities is not clearly defned nor well understood. Terefore, the authors resorted to other search techniques, mainly manual search, to strive for full coverage of the research area. We also applied (1) backward and (2) forward snowballing, which refers to using the list of references or citations of the primary studies to identify additional candidate studies [63]. Te guidelines and processes elaborated in [61] were shadowed to optimize the search results. Although such manual approaches may be thought less efcient, the literature backs up their usefulness in pinpointing relevant articles [61]. In our case, the newly discovered articles were added to the list of articles.
Overall, the search exercise collated 698 candidate studies, where 661 articles emerged from the protocol-based search (i.e., academic databases) and 37 supplemental articles from manual searches. Figure 3 succinctly sums up the search and selection results. It is worth noting that the supplementary articles were derived from scanning the relevant articles (i.e., snowballing) and hand searching to reduce the bias during the selection process. In Figure 3, we bundled the manual search results with the total candidate articles to facilitate reading the fgure.
Due to the versatility of search options provided and the limitations imposed by each academic database, we had to use a fuse of techniques, where sometimes we had to (1) split the queries into small search phrases and (2) search the title, abstract, author-specifc keywords, or complete article to identify a reasonable number of articles. We provide more insights into the queries that we used to identify the most relevant articles from each database.
For ScienceDirect, service composition aspects were searched in the title, abstract, or author-specifed keywords, while the disability aspects were searched in all parts of the articles (metadata and full text). In IEEE Xplore, we deployed our queries using AND/OR operators and searched the metadata and full text of articles to obtain our desired results. For the Springer database, we searched for service composition aspects in the titles of the articles, while the disability aspects were searched in all parts of the articles. For the ACM digital library, we searched for service composition aspects in the abstract of the articles only and for disability aspects, we searched anywhere in the articles. For the ISI Web of Science, we used the Query builder option in the advanced search and submitted queries for service   Computational Intelligence and Neuroscience composition aspects and disability aspects to search all parts of the articles. Finally, for Google Scholar, service composition-related concepts were searched within the titles of the articles, while disability aspects were searched in the full text of the articles.

Selection Criteria.
When we implemented the search strategy, we retrieved 698 possible articles. Our subsequent task was to refne the results into a subset of studies that assisted in answering our research questions. Terefore, explicit inclusion and exclusion criteria were defned and applied to shortlist the relevant articles. We specifed and applied fve inclusion rules and three exclusion rules, which are represented via the dotted rectangles in Figure 3. In summary, only peer-reviewed, English-written journal and conference articles published between 2010 and 2022 were fltered from the six bibliographic databases. Next, the authors carefully read the title and abstract of each article to decide on the relevance of the candidate studies. Tey had to read the full text in certain instances, particularly when the inclusion decision was not possible from reading the title and abstract alone. For any study to qualify for inclusion in our review synthesis, it must satisfy two conditions. First, the study ought to investigate service composition or integration models, techniques, or approaches. Second, the study must target the creation of software services for people with disabilities regardless of the form of disability.
Research articles that were duplicates and appeared in multiple academic databases were disregarded. In the case of multiple publications from the same authors about the same study, only the latest and most comprehensive article was included in the analysis. Review and survey articles were omitted from our SLR. Furthermore, articles that focus primarily on other aspects of the software service lifecycle, such as discovery, selection, invocation, and monitoring, were excluded.
Upon applying the inclusion and exclusion criteria (in Figure 3), the pool of articles was reduced to a reasonable number of 38 relevant articles, constituting the fnal list of our SLR. Indeed, this number of articles is in the range of existing service composition surveys (e.g., [47,48]).

Quality Assessment.
We carefully devised the assessment criteria to evaluate the quality of the primary studies that qualifed for our SLR. Quality assessment is conceived as a crucial appraisal strategy of the collected evidence since it helps to confrm the strength, thoroughness, and credibility of the selected studies [64,65]. In total, we devised 17 criteria covering distinct research aspects (e.g., research questions, methodology, data, user testing, and fndings) to realize an exhaustive quality assessment of the selected studies. Te criteria listed in Table 3 were evaluated on a three-point scale using the following scores (Yes � 1, No � 0, and Partial � 0.5). Te quality score for each study was calculated by totaling the individual scores awarded to each of the 17 criteria.

Data Extraction and Synthesis.
In our analysis of the articles, we developed a comprehensive analysis form following the frameworks proposed by [6,22,49], covering major aspects of service composition. All results are discussed thoroughly and summarized in the proposed taxonomy in the results section. Te form was constructed to  guide and aid the extraction process of the required data to answer the SLR questions. Figure 4 lays out a high-level structure of the data extraction form, which incorporates six main sections. Tese sections encapsulated general information about the articles, the focus of the studies, target disabled users, service composition models, type of services, and quality assessment. Each of these subsections incorporated further details about SOA. Remarkably, the service composition models subsection was constructed based on the recommendations ofered in recent works, precisely in [6,22,49]. Te authors in [22] advocate using their analysis taxonomy, which was constructed to acquire an in-depth understanding of service composition models, techniques, and tools. Although the process was exhaustive, the extracted data assisted us in answering our research questions. It is worth noting that the form was refned several times through pilot data extraction before reaching the fnal state presented in

Results and Discussion
Tis section succinctly synthesizes the main fndings and evidenced observations from the primary studies of service integration for disabled persons. Te results are presented in fve subsections as follows.

Service Composition Landscape.
We began by inspecting some general information (i.e., publication timeline, publishers etc.) about the articles selected in our SLR. Figure 5 shows that most studies (76%) were published in four popular venues, namely Springer, IEEE, ACM Digital Library, and MDPI. Te remaining studies (24%) were scattered across other academic publishers (e.g., Taylor and Francis, ScienceDirect, and Hindawi). Out of 38 peerreviewed articles, 20 were presented at conferences (52%), and 18 were published in journals (48%). All articles appeared in distinctive venues (i.e., no two articles were published in the same venue). Figure 6 shows the trend of research articles published over the past 12 years (from 2010 to 2022). Te graph shows an apparent decline in the number of published research eforts concerning SOA solutions tailored toward assisting disabled persons in the past four years. Tis phenomenon is intriguing, particularly with the prevalence of modern enabling technologies such as IoT, machine learning, robotics, and augmented reality technologies. Figure 7 shows that 15 articles (39.47%) were cited at least 10 times. Te highly cited studies (i.e., >40 citations) that were arranged in descending order include [66] in Enterprise Information Systems, [67] in IEEE Network, [68] in ACM Transactions on the Web, [69] in Sensors, and [70] in IEEE International Smart Cities Conference. Tese topcited studies were published between 2012 and 2017, where [66,67] were arguably the most impactful works in service composition for accessible services. Te authors in [66] explore the domain of in-home healthcare services that are based on Internet of Tings technology. With the help of a codesign framework, the authors attempt to integrate devices, services, and information systems to improve the quality of life of the elderly and disabled users. However, the authors in [67] emphasize the notion of cloud networked robotics where the integration of standalone robots and their functionalities is accomplished to provide seamless support for the daily activities of people with varying disabilities (e.g., elderly and disabled). Six services were considered in the baseline project; however, the presented study focused on touring services for a physically disabled person in a shopping mall.
We carried out a co-occurrence analysis of articles' keywords to understand the common concepts that were researched in our selected articles. Te frst analysis reports the frequency of the authors' keywords used in the articles. A total of 171 keywords were collated from the 38 articles (on average, 4.5 words per article). Te most recurring keywords in the articles included service composition (16 times), service (13 times), Internet of Tings (8 times), objects (8 times), SOA (6 times), user interface (6 times), web (5 times), Table 3: Quality assessment criteria (17 items).

Quality criterion (QC)
Description of criterion QC1 Quality of publication QC2 Well-defned research questions QC3 Clearly explained contributions QC4 Clearly described motivation scenario of service composition for disabled people QC5 Clearly presented composition framework/algorithms QC6 Accessibility design followed (i.e., needs of people with special needs considered) QC7 Conformance to technical standards, languages, or specifcations of service composition QC8 Clearly stated QoS parameters, including nonfunctional properties of service composition QC9 Comparison with state-of-the-art service composition methods QC10 Dataset (of services) clearly defned QC11 Framework/solution proposed supports the verifcation and validation of service compositions QC12 Composition approach accommodates multiple types of disability QC13 Composition approach tested in diferent disability contexts QC14 Composed services/composition tested with end-users QC15 Treats to the validity of fndings discussed QC16 Research implications (i.e., theoretical and practical) recommended QC17 Research limitations/challenges of the study clearly highlighted 10 Computational Intelligence and Neuroscience ambient assisted living (3 times), and context-aware (3 times). Moreover, we created a term co-occurrence map for the terms appearing in the titles and abstracts of our fnal articles. Figure 8 depicts all the concepts, their frequency, and the co-occurrence between these concepts (in the form of links). Tree cohesive clusters were evident from the visual representation, namely, (1) services and service composition, (2) users and their requirements, and (3) IoT solutions and devices.

Target User Groups.
Tis subsection describes the characteristics of users with diverse disabilities who were targeted in the selected studies through the design and/or validation of appropriate SOA solutions. Moreover, it unveils the genre of software services and user interaction mechanisms used to enable the consumption of composite services. Our results show that it was not uncommon for one research article to compose services that assist multiple user groups. 26 articles targeted more than one user group; for instance, the authors in [71] addressed three user groups: the elderly, users with visual impairments, and users with mental impairments. Each study targeted between one and four types of disability. It was quite noticeable that elderly users were also considered when creating assistive composite services. Te older generation represents a viable target group for assistive composite services since older adults sufer a drastic reduction in their cognitive and physical abilities as they age. Figure 9 depicts a taxonomy of the highly recurrent user categories that were targeted in the SLR studies, including (1) people with disabilities (appeared in 22 studies), (2) elderly persons (20 studies), (3) people with cognitive impairments (12 studies), (4) people with motor impairments (11 studies), and (5) people with sensory impairments (7 studies). Te "People with disabilities" category represents a general group where the authors did not specify the type of disability targeted in the study, i.e., the solution was aimed at   people with special needs. Cognitive impairment represents a loss of functions related to the brain processes (e.g., memory, attention, and understanding). In contrast, sensory impairments represent disabilities linked to the vision (i.e., blind) or/and hearing (i.e., deaf ). Te ethnicity of target user groups was mainly European (36%) and Australian (3%), while the remaining (61%) were unspecifed. Inspecting the countries reported in the articles, the end-users who were involved in the service integration studies were from Italy [68,[72][73][74][75][76][77], Greece [77][78][79], UK [77,79], and Sweden [66,79], as depicted in Figure 10. It is worthwhile to note that three studies involved users from multiple countries (i.e., [77][78][79]). Based on age, three classes of user groups emerged from the syntheses, specifcally the elderly and adults (11 studies), elderly (7 studies), and elderly and children (2 studies, specifcally [80,81]). However, 17 articles (44.73%) remained anonymous about the age group of their end-users.
Te authors in [82] emphasize the need to advocate for disabled persons in the design process to cocreate socially inclusive systems. However, we observed that only eight studies (22.22%) opted to consult with endusers regarding their universal design decisions. Te number of users engaged in the studies varied considerably between 1 and 1958, as depicted in Table 4. Notably, the study [79] engaged a high number of end-users (1958 users) in the evaluation process of their service composition approach. Te proposed method combines ambient assisted living services and prepares them for consumption by older people with cognitive impairment, especially in emergencies. Only two articles [68,85] gave gender-specifc information.
When we inspected the modalities used for interaction between the users and composite services, 19 studies (52.77%) reported the implementation of multimodality to facilitate the use of services. In comparison, three studies relied on unimodality (a single interaction modality). Conceptually, multimodality refers to using diferent modes (e.g., aural, visual, and haptic) to enable interaction with assistive services [86]. 14 articles were unclear about how interaction can be conducted. Te next natural question that we attempted to discover concerns the techniques and devices used during the interaction process.
Tree input techniques and devices emerged as popular choices among the selected studies (see Figure 11). Both speech recognition commands and tactile interaction appeared in 14 studies (36.84%). Moreover, (smart) mobile phones or devices were reported in 11 studies. In six studies and fve studies, wearable technologies and brain control interfaces were mentioned. Only a few studies used body movements (i.e., [87,88]) and eye-tracking technology (i.e., [74]) to enable user interaction.
Screen displays were the primary type of output device for delivering information and feedback to people with disabilities (19 studies). Tis was followed by speech (9 studies) and notifcations/alerts (9 studies). Text messages and vibrations were used less frequently, as shown in Figure 12. One of the critical aspects that we investigated relates to the accessibility of SOA solutions and the services proposed in the selected studies. Strikingly, only 18 articles (47.36%) considered accessibility features of the composite services to enable the inclusion of people with disabilities. Nine studies did not consider the accessibility of their services, while another nine articles remained unspecifc about designing for accessibility. Based on the recommendations of [82], accessibility design must be inspected from two perspectives, i.e., functional and social aspects, to create socially accessible designs. Te functional factors investigate the technology features such as information architecture that improve the technical quality of a system. In contrast, the social design factors impact user perception and use of the system, such as personal safety, respect, appropriateness, emotional support, and social appeal.
Twenty studies highlighted functional features that should be considered in developing assistive technologies. In general, these studies suggested the use of multiple input and output modes of interaction (e.g., [74,89,90]), the adaptation of interfaces to ft diverse device characteristics (e.g. [73,91]) and user profles (e.g., [81]), and the simplicity of the user interface (e.g., [78,79,83]). Most of the functional needs were related to the interaction and user interface of composite services. On the other hand, we were surprised to Computational Intelligence and Neuroscience discover that only fve studies (13.15%) incorporated social needs when mashing up services (see Table 5).

Services and Domains of Composition.
Next, we inspected the targeted application areas in the selected studies. Two major domains emerged from the analysis. 16 studies provided composite services to enable the realization of smart homes (e.g., [76,93]), spaces (e.g., [88]), and cities (e.g., [70,73]), while another 12 studies aimed at creating composite services to support the concept of smart assisted living (e.g., [84,[94][95][96]) as depicted in Figure 13. Other  emerging felds of application include smart tourism (e.g., [72]), smart health (e.g., [66,97]), and smart transportation (e.g., [74]). Te integrated services in these domains were accessed and consumed using web systems (18 studies), smart mobile devices (15 studies), and sensory environments (11 studies). Sensory environments refer to places and spaces designed to support the processing of sensory information through multiple senses to enable disadvantaged groups, such as children with autism [98], to partake in several activities and improve their well-being. In two distinguished instances (e.g., [67,92]), services were invoked via robots.
Overall, two types of services were composed, namely, software and web services (28 studies) and IoT services (16 studies). Most compositions included heterogeneous services (33 studies, 86.84%), while only fve articles were unspecifc about the composed services (e.g., [83,88,89,95]). Only two articles made their services publicly accessible (e.g., [81,99]), while the remaining 34 studies completely concealed the details of their services. Te composite services were accessed through a myriad of interactive devices ranging from smartphones to devices (27 studies) such as PDAs, desktop computers (7 studies), braincontrolled devices (3 studies), wearable technologies (3 studies), smart wheelchairs, and smart sticks (2 studies). However, seven studies remained vague about which devices are used by disabled persons to beneft from the composite services.
In the next step, we inspected the languages used to describe the functionalities of services. Only  Italy 36% Australia 5% Figure 10: Countries of people with disabilities involved in the studies. Te fgure shows that European countries (with Italy in the lead) represent the majority.   machine interpretation [103]. Such abstraction of semantic descriptions achieves critical benefts, including interoperability, automation of service lifecycle tasks, and improved performance.
A myriad of ontologies is proposed to remove the barriers to using technologies for disabled persons ( [104,105]). Ontological models and taxonomies may be used to express diverse aspects of a disability, including the type of disability, interaction context, abilities, and policies, among other entities. However, a recent review revealed a lack of applying semantic web technologies to realize wider software accessibility [106].
With regard to messaging protocols, fve studies (i.e., [75-77, 83, 87]) used Simple Object Access Protocol (SOAP), two studies (i.e., [93,96]) used Representational State Transfer (RESTful), and 31 articles remained unclear about the way communication occurs between the composed services. Te SOAP and REST communication protocols difer since the SOAP standard exposes the service logic and operations using dedicated XML-based interfaces. In contrast, RESTful services use HTTP to access web resources and facilitate communication among services. Tere are various reasons why world-renowned companies prefer RESTful architecture, primarily owing to the ease of integration with other resources, speed, and decreased bandwidth. Moreover, only one study [77] provided Universal Description, Discovery, and Integration (UDDI) support to enable the registry and discovery of accessible services by clients and composers.

Studies
Social needs [85] Personalization of services as per the culture and lifestyle of disabled users [77] Reduction of social isolation [83] Autonomy to survive in isolation; social inclusion [80] Individual profling to help users in social contexts [92] Social function services, e.g., inclusion; psychological comfort

Service Composition Approaches.
Te composition brings various types of software components together to serve business requirements. In our SLR, the services integrated ranged between data (20 studies), application logic (16 studies), and user interface components (1 study [81]). Te services communicated mainly by using SOAP (5 studies), REST (4 studies), and Open Service Gateway Initiative (OSGi) (1 study, [100]). In six articles [71,72,[75][76][77]87], services were described using Web Services Description Language (WSDL). Other less frequent description languages that were reported one time only included HTTP Rest [96], BPEL [89], and deployment descriptors [100]. However, 27 articles did not reveal details about the languages of their services. Data between services are exchanged in the form of XML/JSON (14 studies, 36.84%) and Java Objects (1 study).
Most services implemented the pull technology and/or business protocol (22 studies) as their primary interaction style. In the pull process, service clients periodically submit requests for the services. On the other hand, the push process sends updates to the clients as information becomes available. However, business protocols outline the rules for sending or receiving messages between services. Service selection for compositions may take place during design time, deployment time, or runtime. In our pool of studies, it was more frequent for services to be selected during design time and/or runtime (25 studies, 65.78%) than deployment time (e.g., [72]), as shown in Figure 14. Only [95] did not indicate the time-of-service selection.
Te studies targeted creating three types of composite applications, i.e., mashups, business processes, and workfows. In essence, mashups fuse a mix of web content, resources, and applications from multiple sources into a single web application. Business processes refer to the logic and steps that are responsible for executing business rules and activities of a service. Workfows, however, refer to a repeatable and sequential set of tasks, defned as part of a formal diagram, to achieve a particular process. In our SLR, mashups (14 studies) were the most produced type of applications, followed by business processes (14 studies), and workfows (9 studies), as depicted in Figure 15.
Text approaches (18 studies, 47%) were used to represent compositions, followed by visual notations (7 studies, 18%), as shown in Figure 16. Te text notations were highly reliant on XML and code-based approaches. In contrast, the visual notations combined diagrams and spreadsheets to represent services and their underlying logic. Four articles (i.e., [73,75,76,110]) reported the use of hybrid (text and visual) notations. Nine articles remained anonymous about the composition notation they employed.
Next, we inspected the composition paradigms that the researchers adopted. According to [22], a composition paradigm is a programming approach that uses dedicated principles to solve problems. Our synthesis demonstrated the prevalence of functional (13 occurrences) and rule-based (13 occurrences) approaches (as shown in Figure 17). In functional paradigms, services are represented in the form of stateless functions. In rule-based paradigms, services are represented as conditions and business rules that map conditions into actions. Tis was followed by the fow and event condition action (ECA)-based approaches (7 occurrences each). Flow-based paradigms expose services as black boxes connected within a graph, and they usually include control and data fow approaches. Te query and scriptbased approaches were the least favorite paradigms among the authors. Eight studies (i.e., [67-69, 71, 83, 85, 94, 107]) mixed more than one composition paradigm to integrate services.
We looked at the building blocks of composite services. Reference [22] divides composition constructs into processoriented patterns, data fow patterns, and data transformations. Control fow constructs (24 studies, 66.66%) were used more frequently than data fow constructs (10 studies, 27.77%). Tis is probably justifable since data-fow concepts are usually more difcult to interpret than control-fow concepts [24]. In control fow patterns, the order of executing services and activities is specifed, whereas, in data fow patterns, data passing from one service to the next is defned. Composition concerns that were considered included security (8 studies, [66,75,76,85,91,92,96,100]), quality of services (5 studies, [68,71,83,84,107]), and exception handling (3 studies, [68,70,90]). One of the key advantages of SOA is its ability to promote the reuse of artifacts and techniques. Table 6 shows that service components (20 studies) were the most reused artifact, while search and discovery were the most reused technique (16 studies). Nine articles remained completely unspecifc about knowledge reuse.
Concerning the automation of composition, modeldriven composition (16 studies) emerged as the dominant type, followed by synthesis-based (11 studies) and planningbased development (6 studies). Automating service composition refers to the automation of the tasks of composition workfows (e.g., discovery, selection, and binding of services). In model-driven development, software models and formal diagrams, such as UML, state machine, and BPMN, were used to generate code and automate service composition. On the other hand, synthesis-based composition, such as the Roman model, followed a client-based approach by exploiting the behavioral features of services to synthesize new compositions via a community orchestrator [112]. Lastly, planning-based composition focused on rule-based reasoning and reinforcement learning, where the composition was specifed as a set of conditions and preferences. Notably, no study provided tool support during the composition of accessible services.
Accessible services were deployed and executed mainly on premises (26 studies, 68.42%) as opposed to the cloud (4 studies, [71,74,78,110]) or cloud and premises (3 studies, [66,77,92]). Te business process engine (i.e., BPE) was the most reoccurring runtime engine for executing processes and services (13 studies), followed by the service bus (4 studies) and code generation (3 studies). Overall, the service composition tools were developed to be operated by two main types of users, namely, professional programmers or developers (26 studies, 68.42%) and end-user programmers (15 studies, 39.47%), as shown in Figure 18. End-user programmers refer to ordinary people who have no professional software development education or expertise [20]. Naturally, they are nontechnical users or domain experts. Interestingly, [88] presented a service composition framework that automates task planning by service robots in smart spaces equipped with information sensors.
Service composition could be achieved in diferent forms [25]. Te composition view refers to the perspective and focus of the composition approach (i.e., processes and data) [113]. In our systematic survey of accessible service composition, service orchestration (47.36%) and workfow (26.31%) compositions were the most frequent composition approaches, as depicted in Figure 19. Service orchestration is usually regarded as a single party. It enables the management, streamlining, and execution of business processes by invoking the correct services for the diferent processes.
However, workfow compositions are based on a progressive fowchart-like diagram of tasks and actions needed to accomplish specifc goals. Workfows might also include some technical descriptions while remaining are platform-independent. Mashup composition (23.68%) comes next in the list where users can create a process (e.g., [69,71,83,84,94,99]) or data (e.g., [72,77,88]) web applications using a dedicated dashboard.
Most studies (81%) implemented a framework-based mechanism to compose services. However, there were a few alternatives to this composition mechanism; for example, [79,107] implemented heuristics to compose services, while [80] used agents to facilitate the composition tasks. Half of the articles (50%) did not provide details of their service composition algorithms and approaches. In the remaining studies, multiple objective approaches [69,75,76,78,83,84,88] and AI/ML techniques (e.g., collaborative fltering [79]), reinforcement learning [107], and graph plan algorithm [93] were used the most (41.44%), except ant colony optimization [74] and the fuzzy preference model [110]. 30

5.5.
Accessible Service Composition Taxonomy. Taxonomies represent a classifcation structure of knowledge or key concepts in a particular domain [116]. Arguably, taxonomies contribute various advantages, including the organization of knowledge into a logical structure, categorization of concepts/classes, and creation of a common vocabulary of the topic concepts [116]. However, developing efective taxonomies is not an easy task. Te literature proposes several ways to represent taxonomies, including a hierarchy, tree, paradigm, or faceted analysis. Our research argues that the proposed taxonomy follows a facet-based structure where diferent concepts are represented independently through multiple perspectives. Tis taxonomy type can be easily altered and/or extended in subsequent research studies.
We developed the so-called "accessible service composition" taxonomy in this SLR. Tis taxonomy concerns the classifcation of several aspects of service composition for people with disabilities. It aims to assist the developers to create accessible services that ft the diverse demands of disabled people. We opted to develop one broad taxonomy of the fndings instead of creating individual taxonomies for each research question. Te aspects of the taxonomy illustrate the answers to the frst (user groups and assistive services), second (user-centered design of composite services), and third (composition approaches) research questions posited in Section IV. However, we split the taxonomy into two fgures for readability and clarity purposes (Figures 21 and 22). In particular, the taxonomy classifes and arranges the types of disabilities and services, user groups, user interaction mechanisms, and service composition methods covered in the selected articles. Te taxonomy may be extended in the future to incorporate further knowledge and fndings.
One of the key contributions of our survey is the development of an accessible service composition taxonomy for people with disabilities. Te taxonomy is designed purposefully to be concise, including a limited number of classes and characteristics, since an overly complex taxonomy would be hard to understand and use and, therefore, often be less counter-productive. We argue that our taxonomy brings about several advantages such as the following: (i) Te taxonomy organizes our knowledge of service composition users, languages, technologies, integration approaches, and tools tailored to accommodate the needs of disabled people. Trough this taxonomy, it is also possible to study the relationships between the various classes and hierarchies of accessible service composition. (ii) Te taxonomy facilitates the understanding and analysis of this somewhat complex domain (i.e., service composition) for fellow researchers and practitioners. For example, the main classes of userservice interactions and deployed technologies in the presence of disabilities can be easily perceived from the taxonomy. (iii) Te taxonomy would enable researchers to identify the existing gaps with respect to the conceptualizations and defne novel research directions in this area (i.e., service integration for disabled users). (iv) Researchers in similar or related felds (e.g., service selection and composition in cloud computing, IoT) are invited to revise and extend the proposed taxonomy to include other relevant conceptualizations and thereby constitute an all-rounded understanding of service composition in several emerging domains (e.g., cloud computing and IoT) for people with special needs.       Figure 22: Accessible service composition taxonomy-service description, composition approaches, and execution aspects.
Computational Intelligence and Neuroscience were targeted by the accessible SOA solutions, the genre of impairments hindering disabled users, and their involvement in the service design process. Te latter category (service design) presents the functional and social accessibility features that must be implemented to assist people with disabilities to consume assistive services. User-service interaction of swimlane: Tis swimlane details the modes of interaction, input devices, and output devices exploited to enable the interaction between disabled people and assistive composite services.
Assistive services of swimlane: Tis swimlane sheds light on the types of assistive services composed and the technologies used to access the functionalities of those services. We extend this swimlane to include quality of service factors, including the accessibility and functional properties that were considered when evaluating the quality of composite services. Te usual factors for the functional quality of services were assessed such as the response time, cost, and scalability. However, for the accessibility factors, the focus was on the type of disabilities supported and the user interface considerations to facilitate user-service interaction.
End-user composition of swimlane: In this swimlane, we tried to capture the end-user composition aspects, including the users (e.g., professional developers and end-user programmers) and tools (e.g., SM4All) that were employed to facilitate the composition of accessible services for people with special needs. Te second branch lists the application areas for which accessible services were created.
Service description of swimlane: Tis swimlane includes two branches. Te frst branch provides technical details about the service languages, messaging protocols, and interaction styles used to implement the assistive services. However, the second branch describes the semantic languages and ontologies used to add semantic annotations about the accessibility aspects of services (e.g., impairment type and user profles).
Execution aspects of swimlane: Tis swimlane incorporates two key branches. Te runtime features of the composition branch list the possible automation models, deployment types, runtime engines used for executing composite services, and the cross-cutting concerns of utilizing the composite services. Te knowledge reuse branch summarizes the artifacts (e.g., types of components, data transformation, examples, mapping rules, and process fragments) and techniques (e.g., search and discovery and recommendation) that were reused during the integration of services.

Quality Assessment of Selected Studies.
Our fnal activity in this SLR pertained to conducting a holistic appraisal of the fnal articles on 17 critical criteria spanning across distinct quality areas as listed in Table 7. We assessed important research aspects and the areas of service composition that are deemed signifcant to the accessibility of services. Only the frst criterion (i.e., publication quality) was scored depending on the ranking of the journal or conference venue in which the study was published. However, all remaining criteria were scored on a 3-point Likert scale, where 0 � NO, 1 � Yes, and 0.5 � Partially fulflled. Scores were totaled across all quality criteria and then normalized according to the following formula, where "S" denotes the normalized score, "min" denotes the minimum score, and "max" denotes the maximum score.
(1) Table 7 sums up and sorts the normalized quality scores of all studies. Te frst fve marked " * '" are the top-rated service composition studies. Moreover, Table 7 reports additional aspects of each study, such as publication venue, type of publication, and the number of citations. Te highly cited articles, e.g., [66,67,82], ranked 8 th , 19 th , and 1 st , respectively.

Key Findings and Open Issues
In this section, we revisit our four research questions and answer them based on the evidence presented in Section V. Before delving into the answers to our research questions, we investigate the main research motivations behind the retrieved studies. Moreover, we synthesize the primary research challenges that should receive the highest consideration from the research community.

Research Motivations.
In this subsection, we outline the key motivations that drove the research works of the selected articles. Te research motivations could be categorized into the following four general groups: (i) Develop context-aware systems: Context identifcation is essential to understand the requirements that empower a system to integrate diverse services to achieve the desired or expected outcome. Various eforts were exerted in literature in this direction, such as ambient assisted living and semantic modeling [95], smart assisted living and semantic modeling [96], and SOA architecture to suggest efcient multimodality paths based on the context [73].   problems spanned across diferent domains and the problems are listed as follows: (i) Mobility or tourism: For instance, a microservices platform was proposed where mobility as a service is provided to elderly and disabled citizens to produce personalized routes [70]. Another similar study [77] proposed a framework that can help users with mobility impairments to perform their day-to-day activities and arrange their intercity travel, where the framework gathers the required contents from already existing web services. (ii) Shopping: Te authors in [108] reiterate the notion of cloud networked robotics where integration of standalone robots and their functionalities is realized to provide seamless support to the daily activities of people with varying disabilities (including the elderly). Six services were considered in the baseline project; however, this study focused on touring services for a physically disabled person in a shopping mall. (iii) Healthcare: Te authors present a service composition platform and tool to integrate IoT services and heterogeneous IoT products to assist seniors in their daily life activities, i.e., aging in place (AIP) and inside homes [93]. It enables users to create composite services through a graphical tool based on existing atomic services by IoT products. A medication reminder service was produced as a prototype to test the proposed system, which gives reminders to old adults about their medications through various devices, i.e., light, speaker, wrist-band, and smartphone. Tis study did not explicitly discuss other genres of disabilities except people with mental disorders.
(iii) Devise service composition and selection strategies: Developing service composition or selection strategies is one of the motivating factors for various studies in the literature. Most of these studies can be categorized into static [47,78,92,94], dynamic [67,68,71,84,107], and hybrid [87,91,93,96,100], based on selection and composition. (iv) Build an integration framework, tool, or prototype: Some existing studies aimed to propose frameworks [80,94] and develop platforms [92], tools [72,81], and prototypes [73,76] to facilitate the integration of multifaceted services. For instance, [92] introduces a cloud-based robotic service platform to assist handicapped and elderly people in a robotfriendly environment remotely.

Revisiting Our Research Questions.
We now respond to our questions in the next section. Further details are summarized in brief in Table 8, which specifes the uniqueness of the studies and lists their key strengths and weaknesses, respectively.

Research Question 1 (Type of Services and User
Interactions). What are the characteristics of assistive services mashed up to support the regular activities of disabled persons? Our selected studies are composed mainly of Web and IoT services to realize the promising vision of smart homes/spaces and ambient assisted living. Remarkably, most articles were composed of smart services, sensors, and devices to assist two end-user groups, namely, seniors and people with various disabilities. Tis makes sense since both user groups share common characteristics (e.g., a decline in their physical and cognitive abilities). It was observed that impaired users accessed services using mainly smart mobile devices. User inputs were provided through voice commands and tactile interactions, while outputs were delivered through screen displays and voice user interfaces. Technically, there was a lack of specifcation of service description languages and standards. Only about 40% of services were annotated semantically. Moreover, there was an evident lack of using semantic technologies to specify and incorporate accessibility features into services. Generally, accessible services were not added to a common discovery repository for reuse by other researchers and practitioners, which resulted in restricted knowledge sharing and reproducibility. Despite the prevalence of cloud technologies, we were surprised to learn that most services were executed in on-premise environments.

Research Question 2 (Inclusive Design of Services).
To what extent are disabled users involved in designing composite services?
Previous works (e.g., [82]) enumerate the benefts of including participants with disabilities in the design and acceptance of accessible and assistive technologies. However, only 22% of the studies in our survey involved end-users in their SOA studies. Moreover, their validations sufered from two shortcomings, (1) the number of participants was relatively small, and (2) gender-specifc characteristics were overlooked in the design process. Te adoption of a user-centered design methodology to create inclusive services was lacking from most studies. Te results are therefore barely generalizable. While the functional needs (e.g., interaction mechanisms) of service design were discussed, the social aspects (e.g., social appeal) of accessible design were largely overlooked.

Research Question 3 (Service Composition Approaches).
What are the service composition approaches and algorithms developed to incorporate the needs of people with disabilities and accessible design guidelines while creating new assistive services?
Te requirements of disabled persons were poorly contemplated and addressed in the selected studies. Although ontologies are well known to enhance interoperability and composition automation, we could not fnd any accessibility-specifc ontology to cater for universal design and accessibility of services. Notably, 38% of the articles did not attempt to model any aspects of disability. Moreover, accessibility modeling focused primarily on the type of disability and interaction modalities. Other important features such as personal preferences, physical context, goals Te proposed model considers context awareness, dynamic service provisioning, and uniform availability of information from heterogeneous devices; machine learning was utilized at multiple phases of service execution (+) Implementation of model based on WoO platform (+) Semantic ontology model is presented (+) A smart assisted living use case is described (−) Approach is not well explained (−) Te use case is simple (−) No comparison is made with relevant approaches (−) Service composition and selection modules and parameters are not explained [70] Te components of the proposed mobility platform are implemented as microservices (+) Concept of mobility as a service is introduced along with crowdsourcing (+) A microservices platform is presented to help elderly and disabled citizens to get personalized multimodal urban routes Te proposed SOA architecture suggests multimodality paths for citizens with reduced mobility and the elderly. Te multimodal paths mix bicycle lanes and bike-sharing services in urban environments (+) A prototype is developed, and a case study is presented (+) User's interface and interaction mechanisms consider the context of use (+) Concepts of multimodality are used (−) Details related to service composition/ SOA are missing (−) Requirements related to users with disabilities are not discussed in detail [87] A unique adaptable algorithm adjusts user interfaces as per the device characteristics, such as speech/aural, and visual/touch, based on the user preferences. Service composition is based on an "online synthesis engine" and "ofine synthesis engine" to support a dynamic and static set of services for users (+) Pervasive intelligent home system (i.e., SM4All) for home automation is presented (+) An XML-based format is proposed to defne services of home devices (+) Adaptive user interfaces are proposed to support varied settings (−) No validation of the proposed architecture (−) No real scenario of disabled persons is discussed (−) No prototype is presented [100] Te presented composition framework exploits service properties to select the most appropriate SOA to enhance the performance. Te framework supports automatic, on-the-fy compositions as well as access to a searchable service directory (+) Support the heterogonous SOA implementations (+) Automatic on-the-fy service composition (+) A case study is presented to validate the results (−) Study is slightly outdated (−) Prototype is not presented (−) disability-specifc requirements are not discussed in detail (−) Diferent interface options are not considered [89] A model-driven engineering (MDE) approach suggests models and related transformations that help form adaptable user interfaces by considering the context of use (+) Model-driven engineering approach is presented to support adaptable user interfaces (+) Interaction of users using various devices such as desktops, tablets, and smartphones is considered (+) Diferent modalities are considered (+) Context of use is considered  Te presented framework works as an intermediary infrastructure to provide access to edge services according to user needs. Te framework enables users to receive composed services based on their preferences (+) Te proposed framework provides access to edge services in a fexible and scalable way (+) Edge services can be composed dynamically (+) User preferences and device characteristics are considered (+) User profling feature is supported (+) Programmers can easily develop new services using the proposed APIs (+) Te framework is validated through experimentation (−) Te proposed framework is not tested with actual disabled users (−) Several issues such as security and performance enhancements are discussed (−) Details related to service composition are missing [77] Web services are discoverable based on ontologies, which are presented for domains such as service, transportation, and tourism (+) Dedicated ontologies are presented to cover various domains such as service, transportation, tourism, personal support, e-learning, and social relations (+) Proposed framework supports the needs of mobility-impaired users (+) Service alignment procedures can be completed with the help of a drag-anddrop GUI (+) Use case scenario is presented (−) Solution is restricted to a single type of disability (−) Device characteristics are not considered (−) Tere is no mention of support for context-aware services (−) User interaction with the services, such as input and output from the services, are not defned [93] Use of Web of Objects (WoO) concepts for assisted living in smart homes. Te presented architecture fnds the status of the elderly automatically and enables developers to ofer personalized services (+) A composite virtual object (CVO) representation model is presented for efcient and scalable service composition (+) Te CVO-based mechanisms help develop and ofer various features and services for smart home environments (+) Te proposed CVO-based mechanisms can be dynamic and semiautomatic or predefned and static (+) Implementation and experimental studies are presented (+) A GUI-based composition prototype is developed and presented (−) Scenarios for various types of disabilities are not presented (−) Comparison with similar works is not carried out in the validation section [81] Te study introduces a service composition tool that adds voice interaction capabilities to mobile applications to enable disabled users to interact using voice commands (+) An extension of an existing service composition tool is presented (+) Vocal interfaces are realized in detail (+) Validation of the prototype is conducted (−) Study is limited to a single tool extension (−) Only voice interaction is considered (−) Study does not present a new service integration framework [90] Te composition framework is based on the AND_OR search concept. Tis novel approach makes the search in the partial policy by considering the goal requests.  Te use of clustering and classifcation mechanisms to automatically learn the service capabilities; the proposed algorithm has the capability to perform mining based on the past behavior of services (+) Te proposed approach supports pervasive environments by providing a plug-and-play context awareness (+) Automatic description of services using observational learning (+) An architecture is proposed to integrate the learning capabilities (+) Smart home scenario is explained to validate the proposed approach (+) Simulation of approach is performed (+) Smart home prototype supports users with disabilities (−) Disability scenarios are not detailed (−) Multiple disabilities are not considered (−) Validity does not cover diferent types of users (−) Te impact of service parameters/QoS on the approach is not discussed (−) Te proposed model does not test the complex relationship between the impact of preconditions and efects [108] Te proposed Web of Objects architecture employs a semantic ontology model to infer knowledge-based intelligence through objects collaboration (+) Knowledge-driven semantic ontology model is presented (+) A prototype is implemented and presented (+) A mall-based scenario is presented for emergency services (+) Example of a handicapped person is discussed to receive emergency services (−) No validation scenarios related to disabled users are discussed in detail (−) Multidisability support is overlooked (−) Comparative analysis against existing models is not carried out [72] AI techniques, knowledge engineering, and information extraction are utilized to support the integration of tourism services; Te uniqueness of the proposed approach comes from the fact that user feedback is considered during the process of automatic composition of web services (+) Incorporation of ML techniques in integrated tourism (+) A tool is developed to retrieve information from the web automatically (+) OWL-S is used for the semantic description of the services (+) User profling and feedback are incorporated during the service composition process (+) A use case is implemented for an actual city in Italy (+) Disabled users are supported for various services (+) Multimodality for service selection is considered (−) Support for multiple disabilities is not mentioned (−) Solution is developed for a specifc project, and validation of its generality is not tested (−) Te tool is not optimized to handle big data, so it might be challenging to accommodate scenarios of big cities [66] Te proposed business-technology codesign methodology is unique in the sense that it combines business and technology aspects in an in-home healthcare solution to provide services to the elderly and handicapped users (+) Field trials are conducted to verify the proposed methodology (+) A prototype is developed (+) End-users are engaged to validate and test the methodology (+) A computational analysis is presented to show the performance of the developed prototype (+) A real-life demonstration is presented (+) Architecture integrates devices, services, and systems (−) Validation scenarios do not include explicit details about the experience of disabled users (−) Diferent types of disabilities are not discussed (−) User interface design is not highlighted (−) Te proposed methodology is verifed for a small in-home setup while complex scenarios are not tested in the study (−) Te proposed methodology is also not been tested in business practices [74] Te approach combines the concepts of mobility navigation lifecycle with the service-oriented architecture and presents both functional and implementation perspectives  Te approach fnds the best service providers for elderly care through a novel environment, called SCoPE, where several parameters, such as service fltering, composition strategies, and service adherence are utilized. Ranking mechanisms are applied to select services (+) A novel service selection approach that provides tailored services based on user needs (+) A common language is used to identify services and needs (+) Te approach integrates diverse services to meet the social and cultural needs of users (+) A personal profle is created for each user in the context of her lifestyle (+) A case study is presented to validate the proposed idea (−) Te proposed approach is not tested on a large scale (−) Validation and experimental studies are not conducted (−) Disabled scenarios are not included in the study (−) Multiple disability support is not considered (−) Input and output modalities are not discussed [80] Te proposed framework builds disability ontologies by retrieving knowledge and by solving the conficts between distributed ontologies; the created ontologies can help people, such as the elderly, disabled, and children, in various services To solve the problems of dynamicity and diversity in the homecare area, the proposed approach utilizes aspectoriented approach to design and implement dynamic-workfow-based service composition (+) Use of aspect-oriented approach to promote dynamicity and diversity (+) Te proposed approach promotes enterprise interoperability (+) User preferences are considered in the composition (+) Te approach enables caregiver organizations to adjust, add or remove business rules to serve the users of services based on their needs (+) Te approach is implemented to show the validity of the idea; (+) Te approach can support elderly users and users with impairments (−) Te approach does not present experimental validations related to disabled and impaired users (−) Te approach does not provide support for complex business rules (−) JavaScript is used, which makes the approach less fexible (−) New changes are hard to implement (−) User input/output modalities in the context of using services are not discussed [88] Te algorithm can achieve service composition with autonomy, time efciency, and good application value in a dynamic environment (+) Autonomy in robotic service composition (+) Handling of emergency situations (+) Identifcation of an optimal set of services for robot task planning (−) Te service composition algorithm is abstract with no details (−) Te algorithm is analyzed only for emergency events (−) Implementation details are not presented efectively (−) Te technique is limited to robotic environments (−) Context information acquisition details are missing [75] Te platform enables users to interact with the system through various means of communication to achieve the target inhouse activities through preordered or ad-hoc activation of a series of multiple services. Multiple services are composed at runtime with given constraints to achieve the desired goals  Te system (i.e., smart home for all) is developed for smart homes to help coordinate middleware and user interfaces to perform various in-house activities (+) A prototype is developed and tested with 31 clients (+) Te framework targets both nondisabled and disabled people (+) Multimodal interaction is supported (−) No systematic validation strategy was adopted (−) Limited experimental analysis (−) Acceptability and usability are not evaluated thoroughly (−) People with disabilities were not tested explicitly [83] A generic model-driven approach is introduced to reuse models and services based on the context of physically disabled users. Generality, reusability, and integration of services and user interfaces make the approach unique Te use of ontology makes the graphicalbased composition framework generic and unique to integrate and reuse heterogeneous IoT products for target activities of people with mental disorders (+) Integration and reuse of heterogeneous IoT products for diferent scenarios (+) Platform is built on top of SOA that support modularization, composition, and model-driven implementation (+) New IoT products or devices can be easily integrated with a graphical-based system (+) Web-based interfaces for service composition tool on top of the proposed platform (−) Lack of systematic validation of the composition model and prototype (−) Unrealistic assumptions were considered, e.g., availability of IoT sensors and equipments in homes (−) Absence of comparative results analysis (−) IoT products are not integrated dynamically (−) One service (medication reminder) is supported by the prototype (−) Details of context identifcation in the environment are omitted [68] Dynamicity in services composition based on the context to achieve the desired target; the service composition approach implements context awareness and manages contingencies, leverages heterogeneous devices, and empowers users (+) Dynamic composition of services in smart homes based on the context (+) Service/devices are added or removed on an ad-hoc basis (+) Various quality factors are met, e.g., usability and efciency (+) Te system is tested and validated with end-users (+) A detailed methodology and scenarios are presented (−) Te proposed system is not accessible outside home premises (−) Environmental threats are not discussed (−) Blindness and other disabilities scenarios are not studied (−) Comparative analysis is not carried out with other systems [110] A service-oriented workfow-based mobile cloud middleware framework is presented to reduce computation overhead based on a fuzzy set and weight of context schemes. Te framework enables real-time service composition from heterogeneous proximal pervasive resources (+) Intelligent services selection and migration from mobile to cloud platform on an ad-hoc basis (−) QoS is not considered (−) No validation of the composition model (−) Comparison to similar approaches is not presented (−) Te proposed system is not tested with real users (with any kind of disabilities) [67] Initial work in describing robotic service composition in the context of cloud infrastructure to provide seamless support in daily activities for people with varying disabilities (elderly and disabled) (+) First is to introduce the notion of cloud networked robotics (+) Te concept of generalization of standalone robotic functionalities is used to generate diverse solutions (+) Challenging issues were highlighted in robotics (−) Discussion is limited to an existing ongoing project (−) No concrete solution is provided rather an example scenario is discussed Computational Intelligence and Neuroscience 31 A generalized service composition approach to produce customized assistive tasks for diferent target users based on their profles where abstract workfows are mapped to concrete plans through normative reasoning (+) Semiautomatic approach (through abstract workfows descriptions) (+) Validation scenario with real end-users (+) Reliability and adaptability to uncertain situations (+) Concrete algorithms are defned (−) Small number of end-users (7) are considered for validation (−) Limited type of modalities for interaction (i.e., vocal and text) (−) Deployment seems difcult due to the unavailability of ambient assisted living environment [92] A cloud-based robotic service platform assists handicapped and elderly people using path planning service and resource matching services in a robot-friendly environment. Te proposed approach includes a service-matching strategy to fnd and delegate tasks to suitable robots based on user requests (+) Proposed a universal three-layer robot service platform based on SOA (+) Realization of robotic cloud infrastructure for assisting elderly and handicapped people (+) Resource and computation efcient strategy (+) Semiautonomous robot service provision to disabled people (−) System is not verifed with end-users (−) Algorithmic details of robot path planning and resource matching strategy are missing (−) Diversity of robotic services to be integrated is overlooked (−) Environmental constraints are not defned 32 Computational Intelligence and Neuroscience and plans, and service satisfaction were disregarded in the modeling process. During design and runtime, services were selected to form mashup and business process applications. Te composition frameworks and tools relied mainly on text (code and XML) to create the compositions, possibly due to their ability to express complex logic and operations. Te composition languages were mainly functional and rule-based in nature. Te composition tools were designed for use by software developers and end-user programmers rather than ordinary end-users. SM4All was the most frequent composition tool used to compose universal services. Most of the compositions reused control fow and data fow constructs. Remarkably, no single composition tool supported the development of accessible services via design guidance, performance, and testing.

Research Challenges.
Extracting each study's shortcomings gave us more clarity about the potential issues in accessible service composition. In this subsection, we answer our fourth research question.

Research Question 4 (Open Issues).
What are the open challenges hindering the integration of accessible design within service-oriented architecture solutions?
To this end, we classify and summarize the emerging research challenges into the following fve main themes: (1) service datasets, (2) semantic description of accessibility characteristics, (3) accessible and universal designs, (4) service composition implementation details, and (5) validation of composition approaches. Te below mentioned issues are collated based on our critique of the assembled studies and the authors' perspectives Te system module automatically confgures user interfaces of the target devices based on end-user requirements. Te matchmaking mechanism is generic and scalable, in contrast to the existing solutions based on propositional statements, which is based on ontological representation of preferences of people with special needs, rather than rigid descriptions (+) Validation of the proposed matchmaking strategy for automatic UI confguration by a human expert (+) Scalable approach to meet diverse user demands and heterogeneous accessibility aids (+) Transparency for the developers and end-users (+) Concrete rules are defned with example scenarios (−) Confguration is yet to be explored (−) Confguration solution with matching inference mechanism needs to be improved (−) Proposed system is not tested directly with end-users, but rather evaluated through an emulator (−) Dependency of specifying user profle/ preferences [101] Te proposed model incorporates AI and IoT that can help in the development of IoT applications in a broker-oriented architecture where intelligent agents exchange information through shared memory (+) Extends the BPEL markup language (+) Application scenarios for the disabled have been discussed including healthcare, home automation, and system integration (+) Aspects such as dynamism, limited expressiveness, and the lack of continuity in the development process have been discussed in detail (−) Te model cannot deal with fastchanging interaction structures (−) Te proposed model has been designed solely for the IoT based systems where the expected message trafc will be low than moderate latency requirements (−) Te proposed model has not been compared with other existing models in terms of performance and efciency [115] Te presented architecture is based on planes and layers that help to cater for the aspects related to the technologies used and group the diferent tasks. Additionally, their proposed approach tries to solve the problem of heterogeneity and dissemination of information and services related to tourism services scenarios (+) A unique architecture based on a novel idea of planes and layers (+) Te solution solves the problem of heterogeneity and dissemination of information and services (+) Tey make use of already-established platforms (+) Real scenarios have been used to test the proposed architecture (+) A prototype has been implemented (+) Te idea of social sensing has been explored (−) Te model has not been tested for disability scenarios (−) User input/output modalities in context of using services is not discussed (−) Disabled users aspects have not been detailed or discussed and discussions. Furthermore, we advocate some recommendations to tackle the emerging issues from the studies.
(i) Availability and accessibility: Accessible services' datasets are scarce and often kept private except for a few with inadequate information [66,88,92,111].
Arguably, the lack of datasets of accessible services restrains the reproducibility of experiments and limits the reuse of research resources. Terefore, researchers should publish their assistive services on public service repositories for discovery (e.g., through the UDDI standard), reuse, and extension.
(i) Comprehensive ontology: Te lack of semantic ontologies dedicated to describing accessible services is one of the prominent challenges. Indeed, some disability aspects were modeled in various domains, such as tourism [72,77], smart homes [90,93,112], ambient and smart assisted living [79,80,95,96], shopping malls [108], e-government [71], and smart cities [107]. However, previous works failed to develop a comprehensive ontology dedicated solely to describing the accessibility aspects of services.
(i) Social needs and inclusion: Social challenges, needs, and environment were not considered when designing composite services for people with disabilities. It is essential to go beyond the functional requirements and practical acceptance to operate services and work toward realizing socially designed services that overcome social barriers (e.g., social exclusion and isolation) and achieve social acceptance (e.g., equal opportunities). Few studies showed interest in nonfunctional aspects or social needs such as simplicity with ease of use [83] and social inclusion [77,85]. Terefore, a social model of disability should be developed and integrated within the current SOA tools. (ii) Universal design: Design for accessible services was overlooked [70,76,79,88,96]. None of the SOA tools implemented guidelines for universal design. More research should concentrate on annotating services with accessibility features and guidelines to make them ready for consumption by disabled users in smart environments. (iii) Engagement of disabled people: People with disabilities were usually not consulted in developing SOA solutions and accessible services. Our analysis revealed a lack of focus on the requirement analysis for users with special needs [6, 7, 22, 47-50, 52, 53], except in a few instances in which disabled or older adults were involved at some point in the studies [68,71,79,[83][84][85]. Terefore, the requirements identifed from disabled people were rather poor and/or incomplete. Our research reiterates the recommendation to involve disabled users in the design of universal and accessible composite services. Equally, there is an urging necessity to engage people with diverse disabilities in the design and validation of service composition approaches. Gender-specifc dimensions and needs should also be examined in future studies. (iv) Accessibility of design tools: Te development of an accessibility design tool seems nontrivial because no such support tool exists in the literature. Our research suggests the inclusion of design guidance and accessibility checking in SOA tools. Tis is because service designers and developers lack knowledge and expertise in accessibility design. (v) Accessibility-aware service selection: Service selection during the composition process was mainly driven by the functional quality of service (QoS) factors, such as efciency [70,73,96], scalability [91,93,112], availability [100], and response times [88,94]. In our view, the inclusion of accessibility QoS properties in the selection process would produce universal composite services that meet the functional and social demands of disabled people.
(i) System or prototype development: Te absence of prototypes [73,100] or implementation details [88,91,95] to demonstrate the inner workings of service composition approaches for the disabled users. In various articles, we noticed the lack of technical details concerning the implementation of composition models and algorithms. (ii) Heterogeneity: Tere is a lack of systems that support heterogeneity in terms of services, people, devices, and QoS parameters in a single platform [85,87]. Today, we are surrounded by various devices and sensors in an IoT environment, so it is nontrivial to integrate diverse services in such an environment. (iii) Autonomous composition: Although our focus was on the service composition phase, we noticed the lack of using machine learning (ML) models to facilitate the composition activities. A limited number of studies focused on autonomous service composition approaches utilizing recent developments in artifcial intelligence. In one study, the authors utilized machine learning for dynamic service provisioning [96]. Similarly, the authors of another study proposed an automatic service integration approach based on AI and ML in the tourism domain [72], where they used a static approach that cannot be extended to big cities. Only a few SOAs support creating automatic service composition [100]. Te seamless invocation of services by heterogeneous SOAs remains a signifcant challenge.

(5) Validation of Composition Approaches
(i) Disability scenarios: Te defnition of user-centric and real-life scenarios was missing in most of the articles [69,70,87,95,111]. Tese scenarios and case studies are helpful to facilitate the design and validation of the composition approaches to serve the needs of disabled users. (ii) Feasibility and user acceptance: Evidently, little verifcation and validation of the composition approaches or produced compositions was conducted [69,71,76,110]. We expect that service composition frameworks and prototypes must undergo both technical feasibility and user acceptance (also establishing the trust) testing to be judged as efective and replicable. Te superiority of a new service composition framework should be established through comparative testing against existing frameworks. Figure 23 summarizes the major research challenges discovered by our systematic survey.

Study Limitations and Validity Threats
SLRs are quite rewarding when it comes to building a robust understanding of a research problem and answering focused research questions. However, it is imperative to self-criticize the fndings and refect amenably on the limitations of the current SLR. Admittedly, identifying research articles that present service composition solutions tailored towards users with disabilities turned out to be a practical challenge. Te research team had to vary the search phrases and resort to manual searches to reach a reasonable number of articles thus ensuring breadth. Although a resource-intense process was conducted to fnd every possible article, our analysis results were infuenced by the quality of the research design and fndings reported in the fnal selection of the articles. We discovered that several details (e.g., technical information about the composition approaches) were missing from the primary studies during the data extraction phases. Tis could have an infuence on the main fndings. Another limitation is that our search was limited to six electronic bibliographic databases. On this basis, other academic databases, such as Ei Compendex and Taylor and Francis, were not explored separately. Moreover, our search considered studies published after 2010, which increases the likelihood that we might have missed some important studies because of this decision. Our search was restricted to only peer-reviewed conference and journal articles, which means that interesting fndings from short articles and gray literature might have been ignored. Publication and selection bias are major problems that most SLRs sufer from, and we are no exception. Tis is an outcome of not identifying all available data on assistive service composition.
Treats to the validity of fndings afect the quality of systematic literature reviews. Commonly, these threats may arise from four main sources, particularly (1) missing important articles, (2) bias in selecting research articles, (3) erroneous data extraction, and (4) subjective interpretation of evidence. We applied several measures (e.g., PICO and Kitchenham's methodology) to reduce the efects of these threats. Te research team explored the major research databases using varied terms through pilot tests before embarking on the fnal list of keywords (shown in Table 2). Manual search and snowballing techniques were conjugated with automated search so as to not miss any relevant articles. Te eligibility criteria for inclusion in our SLR were clearly defned to concentrate on the composition of accessible services for disabled people. Te extracted data were revised several times by diferent authors to ensure their correctness.

. Conclusions and Future Directions
To the best of our knowledge, this is the frst research efort to synthesize existing works on service composition for persons with disabilities. Tis research topic merits investigation and continuance due to several reasons. First, the number of disabled people is estimated at one billion persons, which emphasizes the notion of service design for disability inclusion. Second, other disadvantaged groups that share similar profles, such as the elderly population, could be greatly assisted by the same SOA-based solutions. Tird, the prevalence of IoT devices and smart services opens the horizon for unraveled composition opportunities in smart homes and places. Our systematic review analyzed and synthesized 38 distinguished service composition studies that were aimed at empowering users with special needs through the provision of assistive services. Te primary studies were selected by applying a rigorous search process to articles retrieved from IEEE Xplore, Web of Science, Springer Link, ACM Library, ScienceDirect, and Google Scholar. Our selected publications spanned the range from January 2010 to October 2022 and appeared in major computing and engineering publications, such as Springer and IEEE. Best practices of systematic literature review, such as the PICO model and Kitchenham's methodology, were implemented in this survey.
Our SLR produced the so-called "accessible service composition taxonomy," which incorporates seven facets, specifcally the target users with disabilities, user-service interaction mechanisms, assisted composite services, end-user composition, services' description, composition approaches, and runtime aspects of the compositions. Te taxonomy would help composite service developers understand the technical and social requirements of disabled users. Unfortunately, the accessibility of composite services seems to be disregarded, with only a handful of works catering to the needs of people with impairments. Best user-centric and inclusive design practices are not incorporated into the composition of assistive services and technologies. We call upon the research community and practitioners to exert more eforts to (1) engage disabled people in the design of SOA solutions and validation of service compositions, (2) defne QoS for accessible services, (3) consider the social aspects of disabilities, (4) provide tool support during the composition phase, (5) and incorporate machine learning advancements to produce more satisfying service compositions that ft the dynamic circumstances of disabled people.
Our future research activities include the design and development of a comprehensive accessibility ontology to facilitate universal service creation and composition. Te ontology would detail disability types, personal capabilities, personal characteristics, social considerations, functional requirements, the context of use, interaction mechanisms, and assistive technologies. Such ontology coupled with a service ontology would facilitate the dynamic selection of services during the composition process to meet the demands of the impaired users. Moreover, we plan to propose and implement a machine learning-guided service selection framework to satisfy the dynamic demands of disabled persons while considering the context and accessibility profles of users.

Data Availability
Te data supporting this systematic literature review were extracted from previously reported studies, which have been cited. Te processed data are available from the corresponding author upon request.

Conflicts of Interest
Te authors declare that they have no conficts of interest.